Fabrication and characterization of Ta2O5 photonic feedback structures
Introduction
High optical gain and broad gain spectra are attractive properties of organic materials for opto-electronic applications [1], [2]. Combining organic films with highly optimized resonant photonic structures is a promising route towards low threshold lasers in the visible wavelength range [3]. The performance of planar photonic crystal structures can be enhanced by integrating a thin layer of a transparent high refractive index material in order to increase the optical confinement in the photonic structure and therefore the mode coupling. This enhanced feedback by photonic crystals has been predicted and demonstrated for TiO2[4]. To avoid the problems encountered in TiO2 processes an alternative material is preferable. Here, tantalum oxide (Ta2O5) has been chosen for its high refractive index which is only about 10% smaller than the one of TiO2. Furthermore, it shows negligible absorbance in the visible and near infrared spectral range, low scattering losses and is compatible with silicon technology. A complete fabrication process for Ta2O5 photonic feedback structures including deposition, lithography and etching has been developed. Finally, hybrid structures of inorganic photonic feedback structures with an organic gain material have been investigated experimentally and functional vertical-emitting laser devices have been characterized.
Section snippets
Experimental
To fabricate photonic crystal structures in Ta2O5 three key process modules had to be developed:
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Deposition of thin Ta2O5 layers.
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Definition of test and feedback structures by electron beam lithography.
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Transfer of the electron beam defined resist features into the Ta2O5 layer.
Subsequent to the development of these modules functional devices could be fabricated by integrating all modules into a full fabrication process flow.
Results and discussion
The second order laser structures fabricated by the process sequence outlined above have been tested in optical experiments. The feedback structures have been optically pumped by a tripled Nd:YAG laser at 355 nm. Special emphasis has been paid to the lasing characteristic of the second order structure. 1Fig. 7 shows the emission spectra of a photonic crystal laser
Conclusion
Photonic feedback structures have been fabricated in Ta2O5. All key process modules for this fabrication, including the deposition of the Ta2O5 layer, the lithography and the dry etching have been developed and optimized. Using a polymer with high optical gain, single mode lasing has been observed. This demonstrates that photonic feedback structures from Ta2O5 offer great potential for the fabrication of organic laser devices in the visible wavelength range. They are a viable alternative to
Acknowledgments
This work was partially supported by the EU project OLAS (IST-FP6-015034). The authors would like to thank M.C. Lemme and C. Moormann for stimulating discussions. Further, the authors wish to acknowledge M. Sousa and D. Caimi for help with the ellipsometry and the preparation of the organic layers and H. Hahn for his skilful help during sample preparation and processing.
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